Electrophysiological experiments with microelectrodes have been performed on chick muscle cells grown in tissue culture and on identified neurons and molluscs. Using the voltage clamp technique the membrane conductances underlying slow membrane potential responses in these preparations have been investigated. Studies on the muscle cells have concentrated on the voltage and time dependent characteristics of the chloride conductance underlying the chloride spike and on the relationship of this spike to the caffeine contracture. Studies on the molluscan neurons have concentrated on the membrane conductances underlying both the spontaneous pacemaker activity and the epileptoform activity found in these neurons. These experiments have focused mainly on the role of the cation (Na ion, K ion, Ca 2-ion) conductances in the generation and control of both types of electrical activity. In addition, the effects and the mechanisms of action of various drugs and hormones on such activity have been investigated.